In general, it is well known that multilayer ceramic substrates have good plasma resistance, oxide-resistance and chemical resistance as well as good electrical insulation. Thus, the multilayer ceramic substrate has been widely used in place of metal in various fields of an electric industry so as to compensate for the physical and chemical drawbacks of the metal. Particularly, the multilayer ceramic substrate has been much more widely used in place of the meal for manufacturing semiconductor devices. For example, the multilayer ceramic substrate has been used for an electrostatic chuck for securing a wafer by an electrostatic force thereon and a ceramic heater for heating a wafer to a high temperature in various processes for manufacturing the semiconductor devices.
The conventional multilayer ceramic substrate is usually manufactured as follows. A plurality of unsintered ceramic sheets is stacked together with one another and is pressurized to be secured to each other. Then, the bundle of the secured ceramic sheets is sintered to a sintering temperature, to thereby manufacture the multilayer ceramic substrate. Particularly, when the multilayer ceramic substrate is used as the electrostatic chuck or the ceramic heater, an electrode layer (or an electrode pattern) may be interposed between the unsintered ceramic sheets and a wiring electrically connected to the electrode layer is exposed outwards through a cavity. An electric power is transferred to the electrode layer through the wiring and the electrode layer generates the electrostatic force or the heat.
FIG. 1 is a cross-sectional view illustrating a processing step for a method of manufacturing a conventional multilayer ceramic substrate having the cavity.
Referring to FIG. 1, a plurality of unsintered ceramic sheets 1 is stacked on each other and an electrode layer 8 may be formed on the unsintered ceramic sheets 1. Then, a plurality of unsintered ceramic sheets having holes 2 for the cavity 3 is stacked on the electrode layer 8. The plurality of the unsintered ceramic sheets is pressurized to each other to thereby form a sheet stack 4 in which the neighboring ceramic sheets 1 are secured to each other. Finally, the sheet stack 4 is heated to a sintering temperature to thereby form the conventional multilayer ceramic substrate having the cavity 3.
Particularly, the pressurization to the unsintered ceramic sheets 1 is usually performed by using a pair of flat molds 5 and 6 that are positioned at a bottom and a top of the sheet stack 4 in parallel with each other. Therefore, there is a problem in that a bottom of the cavity 3 is difficult to be pressurized while pressurizing the sheet stack 4. That is, the electrode layer 8 exposed through the cavity 3 does not make direct contact with the flat molds and thus is difficult to be pressurized in the pressurization process. Therefore, the ceramic sheets 1 tends to be separated from each other and the electrode layer 8 and the ceramic sheets 1 are likely to be deformed.
For those reasons, there has been suggested that a protrusion protruded into the cavity 3 be installed to the flat mold. However, the correct insertion of the protrusion to the cavity 3 generally requires respective flat mold corresponding to the individual cavity in accordance with the shapes and structures thereof and thus necessarily requires an additional alignment process for aligning the protrusion with the cavity 3, which causes operational inconveniences of the flat mold and hardship to a quick treatment of a manufacturing apparatus. In addition, when the protrusion is not accurately matched with the cavity 3, the external pressure applied to the flat mold is likely to be difficult to be exactly transferred to a bottom of the cavity 3. Thus, the pressure to the bottom of the cavity 3 is so weak that the ceramic sheets 1 are not sufficiently pressurized to each other or the pressure to the bottom of the cavity 3 is so strong that the electrode layer 8 is likely to be deformed.
Further, there has been suggested that a plurality of the ceramic sheets 1 be pressurized into the sheet stack 4 and a recess be formed on the sheet stack 4 by cutting off a portion the sheet stack 4 as the cavity 3. However, the partial cut-off of the sheet stack 4 tends to cause damage to the electrode layer 8 and the ceramic sheets 1 around the cavity 3.
Accordingly, there is still a need for an improved method of manufacturing the multilayer ceramic substrate in which the cavity can be easily and effectively formed in the sheet stack.